A conventional high frequency receiver is described with reference to drawings. FIG. 12 is a block diagram showing portable equipment using a conventional high frequency receiving device. A signal input to telephone antenna 1 is input to an input-output terminal of switch 2. Switch 2 switches a signal between transmission and reception. The signal input to telephone antenna 1 is input to telephone receiver 3 via switch 2, and further converted into an intermediate frequency signal to be output from telephone receiver 3.
The intermediate frequency signal output from telephone receiver 3 is input to demodulator circuit 4. The signal demodulated in demodulator circuit 4 undergoes error correction in decoder circuit 5, and then is input to voice output part 6 that is one of voice output devices or image display part 7 that is one of display devices. Voice output part 6 and image display part 7 convert a digital signal decoded in decoder circuit 5 into an analog voice signal and an analog image signal so as to be output as a voice from a speaker and to be displayed as image information on, for example, a liquid crystal display device.
The high-frequency receiver shown in FIG. 12 includes input key 10 and voice input part 11 that is prepared as one of voice input devices. These include a so-called microphone and a circuit for driving this microphone. Output signals from input key 10 and voice input part 11 are input to encoder circuit 12. An analog signal output from voice input part 11 is converted into a digital data signal via encoder circuit 12. Furthermore, based on the instruction from input key 10, encoder circuit 12 generates a digital data signal. A digital data signal output from encoder circuit 12 is input to oscillator 13 and then modulated so as to be generated as a transmission signal. This transmission signal is amplified to a predetermined amount of electric power by power amplifier 14 and input to an input terminal of switch 2. An output signal from power amplifier 14 passes through switch 2 and coupler 32 and is released into the air from antenna 1.
A high frequency signal having a frequency of 470 MHz to 862 MHz is input to TV antenna 21. The high frequency signal coming into TV antenna 21 is input to TV tuner 22. TV tuner 22 includes notch filter 23, local oscillator 24, mixer 25 and demodulator circuit 26. To local oscillator 24, PLL 24A is coupled. To notch filter 23, a high frequency signal input to TV antenna 21 is input via coupler 35. Notch filter 23 attenuates a frequency of 880 MHz among high frequency signals. An output signal from notch filter 23 is input to a first input terminal of mixer 25. To a second input terminal of mixer 25, an output signal from local oscillator 24 is input.
An output signal from demodulator circuit 26 is input to decoder circuit 27. Decoder circuit 27 corrects error of a demodulated television broadcasting signal. The output signal from decoder circuit 27 is input to voice output part 6 and image display part 7, respectively.
In such portable equipment, the difference between a frequency of a transmission signal from a transmitter and the upper bound frequency of a receive frequency band of TV tuner 22 is about 18 MHz, which is relatively narrow. Notch filter 23 allows 862 MHz that is the upper bound frequency of a receive frequency band of TV tuner 22 to pass. Furthermore, since a transmission signal is such an extremely high level as about +33 dBm, a frequency of 880 MHz, that is, the frequency of this transmission signal cannot be attenuated sufficiently by notch filter 23.
Therefore, between antenna 21 and TV tuner 22, interference removing device 31 is coupled so that a transmission signal of a transmitter input via the antenna is not input to mixer 25.
Interference removing device 31 includes coupler (directional coupler) 32, variable attenuator 33, variable phase shifter 34, coupler (directional coupler) 35 and controller 36. Coupler 32 is coupled between telephone antenna 1 and switch 2, distributes a transmission signal and inputs the distributed signal into variable attenuator 33. An output terminal of coupler 32 is coupled to variable attenuator 33. Variable attenuator 33 is allowed to change the attenuation amount based on an instruction from controller 36. An output terminal of variable attenuator 33 is coupled to variable phase shifter 34. To variable phase shifter 34, an output signal taken out of variable attenuator 33 is input. Variable phase shifter 34 changes the phase of the output signal from variable attenuator 33 based on the instruction from controller 36.
Coupler 35 is coupled between TV antenna 21 and notch filter 23. Furthermore, to coupler 35, an output signal from variable phase shifter 34 is input. Then, in coupler 35, an output signal from variable phase shifter 34 and a high frequency signal input to TV antenna 21 are synthesized.
Between the input of controller 36 and the output from decoder circuit 27, error rate determination device 37 is coupled. Error rate determination device 37 determines the error rate in decoder circuit 27. When the error rate is beyond a predetermined rate, error rate determination device 37 sends out a signal having information indicating that the error rate deviates from a predetermined range to controller 36. When controller 36 receives the signal from error rate determination device 37, it controls variable attenuator 33 and variable phase shifter 34.
Variable attenuator 33 and variable phase shifter 34 cooperatively allow the phase of the transmission signal and the phase of transmission signal that is released from telephone antenna 21 and is input via TV antenna 21 to be reversed to each other at about 180°. By synthesizing these transmission signals by coupler 35, a transmission signal entering a TV tuner from telephone antenna 1 via TV antenna 21 is cancelled to remove the interference by the transmission signal.
As information on prior art document relating to the invention of this application, for example, Japanese Patent Unexamined Publication No. 2000-156657 is known.
However, in such a conventional high frequency receiver, variable attenuator 33 and variable phase shifter 34 is controlled based on the error rate of the signal in decoder circuit 27 determined by error rate determination device 37. However, it takes a relatively long time to carry out demodulation in demodulator circuit 26, error correction in decoder circuit 27 and detection of error rate in device 37. In particular, in order to determine the error rate, it is necessary that data of 104 bits or more are determined. This determination of this error rate needs, for example, a time of one second or more. Therefore, with respect to the phase change of a transmission signal input from TV antenna 21 or amplitude change, a variable attenuator controlled by controller 36 or respond to a variable phase shifter are delayed. As a result, rapid change in the phase or amplitude of the transmission signal cannot be followed, so that interference by the transmission signal cannot be cancelled.